OK, cool. I didn't realise I was underexposing so badly.
Yeah, the metering was centre-weighted. They were just quick snapshots while on holiday so I didn't use my light meter.
I always use the film at its box speed, so it looks like the rule of thumb for this film is use it at box speed if the light is "standard daylight" but set it to EI400 or 640 if low light (or switch to manual but for quick work like this, changing the EI seems like a good solution to me). Thanks all.